Water drops in the air mix with CO2, making the rain a little bit sour. This rain erodes rock, washing out all kinds of minerals into the Ocean.
Here, the minerals and carbon form things like corals and shells. When these die, they sink to the ocean floor and form into rocks.
When animals and plants die, their carbon is released back into the air. But if they die under water this can't happen. Instead, they sink to the bottom and form rocks.
The carbon rocks melt and become magma. Until eventually, a volcano shoots it back into the atmosphere.
This slow cycle also creates a balance. Without it, all the carbon would end up in the atmosphere or as rocks.
But there is one more way that carbon can enter back into the atmosphere. You probably know where this is going.